The CDC issued a travel advisory this week warning pregnant women to avoid the one-square-mile area of Miami where most of the 16 known infections occurred.
While the virus, which is primarily spread through the bites of infected mosquitoes, usually causes only minor symptoms like a mild fever and rash -- and often no symptoms at all. But it is particularly dangerous for pregnant women as it has been shown to cause a birth defect called microcephaly in babies.
Heartbreaking photos of babies born with microcephaly began appearing in news reports and on social media last year as the virus spread at epidemic levels first in Brazil and then throughout much of Latin America and the Caribbean.
The photos depict children born with abnormally small heads, the major sign of the birth defect.
Now that the Zika threat is emerging in the U.S., many people have questions about what microcephaly is and what are the risks.
What exactly is microcephaly?
Generally speaking, microcephaly is a condition where a baby's head is much smaller than expected.
To be diagnosed, a baby's head needs to be two standard deviations below the average, or less than the 3rd percentile.
"So what this means is that if you have a head that is smaller than say 97 people out of 100, then you are technically microcephalic," Dr. Aaron Nelson, pediatric neurologist at NYU Langone Medical Center, told CBS News.
Cases of microcephaly vary in severity. The smaller the head and the earlier the brain was impacted during fetal development, the more serious the condition is.
What causes microcephaly?
Zika is not the only cause of microcephaly, and in many babies with this condition the cause is not known. Some babies may develop it because of a genetic problem, while in others it may be the result of certain environmental exposures during pregnancy.
It can develop if the mother was infected with rubella, toxoplasmosis, cytomegalovirus (also called CMV, a type of herpes virus), and now the Zika virus during pregnancy. Infection during the first or second trimester may be especially risky.
In some cases it may also be due to severe malnutrition; exposure to harmful substances like alcohol, certain drugs and toxic chemicals in utero; or an interruption of the blood supply to the baby's brain during development.
Sometimes a baby can even develop microcephaly after birth. "You can have a one-time injury to the brain that can result in microcephaly," Nelson explained. "Children can have a stroke, they can have deprivation of oxygen, they can have traumatic injuries and then subsequently down the road they can acquire microcephaly."
How common is microcephaly?
Microcephaly is overall a very rare condition. According to the Centers for Disease Control and Prevention, state birth defects tracking systems have estimated that microcephaly ranges from 2 babies per 10,000 live births to about 12 babies per 10,000 live births in the U.S.
In Brazil, where thousands of pregnant women were infected with Zika virus last year, some research has estimated that anywhere from 1 percent to 14 percent of affected babies develop the problem.
What is the connection between Zika and microcephaly?
In April of this year, the CDC confirmed what had been suspected throughout the recent Zika outbreak in Latin America and the Caribbean -- that the virus causes microcephaly.
It is the first time in history that a virus transmitted by a mosquito bite has been found to cause birth defects.
The virus can be passed to the fetus during pregnancy or at delivery, but the mechanism by which the infection leads to alteration in the infant's brain's development is not yet known.
It is also not yet known how likely a baby is to develop microcephaly if the mom gets infected with Zika. "It's important to emphasize that not every pregnant woman who gets Zika will have a baby with microcephaly," Dr. Ricardo Lopez, an OBGYN with Orlando Health, told CBS News.
Still, it's important for pregnant women to prevent mosquito bites to reduce their risk.
How does microcephaly affect a child's life?
Babies born with microcephaly can have a range of health problems, depending on how severe their case is. Some function normally, while others may be seriously impaired.
According to the CDC, microcephaly has been linked with the following issues:
- Developmental delay, such as problems with speech or other developmental milestones such as sitting, standing, and walking
- Intellectual disability (decreased ability to learn and function in daily life)
- Problems with movement and balance
- Feeding problems, such as difficulty swallowing
- Hearing loss
- Vision problems
Depending on the child, these problems can range from mild to severe and often span the lifetime.
Can it be treated or cured?
There is currently no treatment or cure for microcephaly. Because symptoms range from mild to severe, intervention options vary depending on individual cases.
All babies with microcephaly require routine checkups to monitor their growth and development, but those with severe cases need medical care focused on managing their health problems.
"Unfortunately, the majority of care is really associated with identifying any problems and dealing with them early," Nelson said. "It's far easier and better for a child to identify any developmental issues early on and get very intense therapy involved, whether that be with language or learning to stand or walk."
Continuously evaluating babies for different types of seizures and administering medication when necessary to control them is also important for children with severe microcephaly, Nelson said.
How can I protect myself and a future child?
The CDC recommends all pregnant women avoid travel to areas where Zika is active, including more than 50 countries in Latin America and the Caribbean listed on its website and the one-square-mile area of the Wynwood neighborhood in Miami.
For people living in affected areas, or those who need to travel there, experts say it's important to take extra precautions to prevent mosquito bites.
"I always tell patients who have to travel [to Zika areas] that you have to take precautions by using mosquito repellent with DEET, trying to stay indoors with air conditioning or screens, and wearing long-sleeved shirts and pants," Lopez said.
Additionally, since Zika can be transmitted through sex, it is important that people who live in or have traveled to active Zika zones and who have a pregnant sex partner use condoms during all types of sexual intercourse or abstain from sex during the pregnancy.
Women and men who live, work, or travel frequently in areas where Zika is spreading -- including the Wynwood area in Miami -- should speak with their health care provider before trying to conceive.
Finally, the CDC recommends that women who have traveled to these areas should wait at least eight weeks before trying to become pregnant. Men who were infected with Zika should wait six months, since the virus may stick around longer in semen.